Junk filtering of Mail on an iPad

It should be done on your Mail Server and not on the iPad.

Gmail have very good Spam Filter.

With the built-in Mail app no, it doesn't support the use/creation of filtering rules - any filtering will need to have been done prior to it reaching the iPad e.g. on your email provider's system.

That isn't a very good solution. I like to scan my spam folder for false positives. I understand that Apple might not want to bring full Apple Mail functionality to the iPad, and that the spam filtering module may take a lot of processing, but it would be nice to have some minimal filtering available. Maybe search for "spam" in the subject line?